Natural vegetation here refers to the natural forest, which in general defines a community of living trees and associated organisms, covering a considerable area, utilizing sunshine, air, water, and earthly materials to attain maturity and to reproduce itself; it is capable of furnishing humankind with indispensable products and services. In general, tropical forests play a vital role in nutrient storage and cycling on land. Nutrients are distributed not only through the trees, but also in the soils, with nutrient cycling playing a key role in the functioning of tropical forest ecosystems and enriching soils, especially surface soils. Any disturbance of forest cover can cause a significant loss of nutrients from the land. The forest types of the country, which in this case are evergreen and deciduous forest types, are influenced by local climate conditions. The simple difference between these two groups is that, in a deciduous forest almost all species of flora in the forest will shed their leaves in the long dry season months. In an evergreen forest there is a higher rainfall and species will fall and replace leaves all year and the forest remains green all year round.

1. The evergreen forest can be subdivided into four types: the tropical evergreen, the
     coniferous, the swamp and the beach forests.

1.1 The tropical evergreen forests occur along the wet belt of the country, where high annual amounts of rainfall of 1,500 mm and up prevail; they are affected by the monsoon. This forest ecotype is further divided into three categories: tropical rain forests, dry or semi-evergreen forests,and hill evergreen or lower Montana forests.

  - The tropical rain forests prevail in southeastern and peninsular regions where
     contact with the monsoon is direct; the precipitation is very high, 2,500 mm and
     up annually. The principal trees in the lower zone, up to 600 m altitude, are

     mostly Dipterocarps, while in the upper zone, 600-900 m altitude, oaks and

     chestnuts are common.

   - The dry or semi-evergreen forests are scattered over the country along the
      valley of low hill ranges of about 500 m elevation, or forming communities
      along streams and rivers. The annual precipitation is between 1,000-2,000 mm.
      The principal trees of this forest ecotype are Anisoptera, Dipterocarpus, Hopea,
      Tetromeles and Lagerstroemia ssp.

 

   - The hill evergreen forests occupy the upper elevations from 1,000 m rising, all
     over the country with the larger percentage occurring in the northwestern
     highlands. This type of forest is known also as temperate evergreen forest or
     the lower Montana forest. The dominant tree species are oaks and chestnuts.

 1.2 The coniferous forests occur in small pockets in the northwestern highlands and the Khorat Plateau (about 200-1,300 m elevation), where poor acid soil is found. The annual rainfall is about 1,000-1,500 mm. The dominant species are Pinus merkusii and Pinus kesiya.

 1.3 The swamp forests occupy areas that are more or less subjected to occasional inundation , and are scattered in the wet regions of the country where the annual precipitation is high (in excess of 2,000 mm). This forest type can be further classified physiographically into two kinds: the mangrove swamp forest and the fresh water swamp.

 1.4 The beach forests occur in areas along the beach, where sand dunes, rocky seashores and elevated seacoasts prevail. The dominant species are Casuarinas equisetifolia and Terminalia catappa

 2. The deciduous forests occupy the dry belt of the country, where precipitation is low (under 1,000 mm annually) and the climate is more seasonal; the soil is sandy and lateritic. The vegetations of these zones are classified as deciduous formation. The tree species of this type shed their leaves during the dry season and tend to develop growth or annual rings. Deciduous forests can be subdivided into three main categories: the mixed deciduous, the dry dipterocarp, and the savanna forests.

 2.1 The mixed deciduous forests occur between elevations of 50-600 m. The principal tree species are Tectona grandis, Lagerstroemia, Terminalia, Afzelia, Xylia, Peterocarpus and Dalbergia spp., most of which are high-value commercial species.

 2.2 The dry dipterocarp forests occupy areas on the undulating peneplain and ridges. The dominant tree species are Shorea obtuse, Shorea siamensis, Dipterocarpus tubercultus and Dipterocarpus obtusifolius.

 2.3 The savanna forests originate from burning, which then forms a deciduous type. It is found more frequently in the northeastern region, where the precipitation is relatively low (from 50-500 mm annually). The savanna is, in essence, a grassland where trees of medium height grow sparsely, forming a very open stand interspersed with thorny shrubs and Bambusa arundinacea.